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All About Sound

All About Sound. Rob Shaffer Stoklosa Middle School, Lowell June 11, 2013. Note: this lesson utilizes the Audacity computer program to create and analyze sounds. It is available for free download from: www.audacity.com. Attributes of Sound. Loudness (volume) Pitch (frequency, tone)

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All About Sound

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  1. All About Sound Rob Shaffer Stoklosa Middle School, Lowell June 11, 2013 Note: this lesson utilizes the Audacity computer program to create and analyze sounds. It is available for free download from: www.audacity.com

  2. Attributes of Sound • Loudness (volume) • Pitch (frequency, tone) • Tonal structure • pure tone or combination of several or many tones • timbre • Duration • Source • Direction • A mechanical wave with a certain propagation speed

  3. Waves • What is a wave • A disturbance in the properties of a medium that propagates through the medium • Transfers energy from one point to another • Particles in the medium return to their previous position after the wave passes • Types of waves • Mechanical waves • Electromagnetic waves

  4. Examples of Waves • Ripples in a pond • Explain wave curvature • Close to the source • Far away from the source • Sinusoidal waves • Concept of phase difference between two sound waves – Audacity example

  5. How is sound created? • Impulsive sounds • Clap two boards together • Longer term sounds • Continuous vibration of a sound source such as a speaker, string, column of air • Have the kids come up and feel the speaker cone vibrate

  6. Sound (in air) is a rapid fluctuation in the air pressure • As a speaker cone vibrates, it pushes on the air molecules, increasing the air pressure. Then it moves the other way, and the air pressure is reduced. • The greater the change in air pressure, the louder the sound is. • How fast the vibration is determines the frequency, or pitch, of the sound • 1 Hertz (Hz) = 1 vibration cycle per second • Healthy ears are extremely sensitive to very small changes in air pressure. • Most people can hear sounds with frequencies between 20 Hz and up to about 20,000 Hz • As you get older, this range is reduced • If you listen to very loud sounds a lot, your hearing range will be reduced at a younger age!

  7. Some units of pressure • The standard unit of pressure is the Pascal 1 Pa = 1 newton / m2 • Normal air pressure at sea level = 1 atm 1 atm = about 100,000 Pa = 14.7 lbs/in2 • If your hearing is good, you can hear sounds with a pressure fluctuation of about 20 μPa (micropascal) = .000020 Pa .000020 / 100000 = .0000000002 atm = 2 x 10-10 atm • That’s about how loud a mosquito is 3 meters away from your ear • A painfully loud sound has a pressure fluctuation of about 20 Pa = .00020 atm = 2 x 10-4 atm

  8. Table of Sound Levels in Air

  9. How does sound propagate? • Mechanical wave: molecules are pushed closer to adjacent molecules, increasing pressure • Sound is a Pressure Wave • Experiment: • Kids line up far apart (too far to reach) from each other • First kid touches the next one • As soon as you’re touched, touch the next one • Time how long it takes for the “wave” to propagate through the entire line • Repeat with everybody closer together • Did the wave travel faster?

  10. Speed of sound in various materials • Depends on how close the molecules are to each other, and how energetic they are (temperature) • Air: 340 m/s (761 mph) at sea level • Water: about 1500 m/s (3355 mph) • Metal (steel): about 6000 m/s (13,421 mph) • In air, that’s approximately 5 seconds per mile • When you see lightning, start counting until you hear the thunder. If it’s 10 seconds, then the lightning struck 2 miles away

  11. Temperature gets colder Pressure increases How does pressure and temperature affect the speed of sound? Ocean Water Air

  12. Direction of Sound • Experiment: • Everyone close your eyes • I walk around the room making noise • Point at me while I move • How do your ears and brain figure out the direction of the sound?

  13. Demonstration of Pitch (Frequency)Science and Music • Listen to tones of various frequencies: • 50, 100, 300, 800, 3000 Hz • Generate 2 tones: 440 Hz, 441 Hz • Show the sine waves • Show how they go in and out of phase • Play one at a time – can you hear the difference? • Play both together, creating beats • This is the most accurate way to tune an instrument • Use microphone, have students sing to try to match the tone

  14. Tone Generation example in Audacity Note: use the Audacity program here, not just this slide. Explain and illustrate the sine waves and how they look at different frequencies, especially very close frequencies.

  15. More with tones • Generate harmonic tones • 440, 880, 1760, 3520 • Play each tone individually • Mix at various gain settings to create sounds with different timbre • More advanced stuff if time permits: • Spectrum analysis of the tones (with Audacity) • Spectrum analysis of voice (with Audacity) • Explain what the spectrum plot shows (amplitude vs. frequency)

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